A brake system of this type is known from German published patent application 33 23 402. The braking pressure is varied in dependence on the wheel rotational behavior by means of electromagnetically operable valves, namely so-called inlet valves and outlet valves, in order to prevent locking of the wheels in the event of excessive brake actuation, on the one hand, and in order to conform the brake force distribution onto the front wheels and rear wheels to the respective braking situation, on the other hand. As is known, the effects of different load conditions of the vehicle, of the static and the dynamic axle load shifts can be compensated for more or less precisely by means of control of the brake force distribution. To preserve the driving stability, it must always be ensured that the rear wheels will by no means lock prior to locking of the front wheels.
To control the brake force distribution, multiple-way valves closed in their inactive position are inserted as inlet valves into the brake lines leading to the rear wheels according to the German published patent application mentioned hereinabove. In contrast thereto, the inlet valves leading to the front wheels are open for the passage of pressure fluid in their inactive position. It is accomplished by clockwise actuation of the valves that an amount of brake slip is obtained at the rear wheels which is always less than that at the front wheels and which, preferably, amounts to roughly 85 to 97% of the front-wheel brake slip. This way, a major contribution of the rear wheels to braking is achieved, while simultaneously overbraking of the rear wheels is prevented.
According to this publication, the inlet valves and the outlet valves, the wheel sensors, the analyzing electronics and the auxiliary-pressure supply system are used for the control of the brake force distribution as well as for anti-lock control.
Further, a brake system is known already from German published patent application 34 40 541 which is equipped exclusively with wheel valves for the electronic control of brake force distribution. Hence wheel valves, namely solenoid valves closed in their inactive position, are inserted only into the brake lines leading to the rear-wheel brakes. Connected in parallel to these wheel valves is a brake force regulator through which braking pressure is supplied to the rear-wheel brakes on brake actuation, which pressure is conformed to the minimum pressure in dependence on the respective braking operation in the event of ideal brake force distribution. This braking pressure is raised to the higher value which optimally conforms to the instantaneous situation by actuation of the wheel valves. This arrangement is based on the consideration that even in the event of current failure or a valve defect, a minimum amount of braking pressure is supplied via the brake force regulator to the rear-wheel brakes so that even on such malfunction the rear axle is able to contribute to braking of the vehicle. Naturally this contribution is in many cases far less than the optimal, theoretically possible value. In other situations, the pressure transmitted via this parallel branch will result in wheel lock.